This invention generally relates to a magnetometer for a torque sensor. More particularly, this invention relates to a magnetometer including several coils disposed relative to each for measuring torque related divergent magnetic fields.
A non-contact, magnetoelastic type torque sensor typically includes a torque transducer element that responds to the application of torque by generating a magnetic field. Such generated or changed magnetic fields are detected by a magnetometer. The torque transducer element typically includes a magnetoelastic material that responds to the application of torque by generating a corresponding magnetic field. The application of torque to the magnetoelastic material creates shear stresses within the magnetized regions causing the direction of the magnet field generated by the torque transducer element to shift from a substantially circumferential direction to a helical direction. The helical shifting of the magnetic field is detected as an axial component of the magnetic field. The axial component of the magnetic field is proportional to the applied torque and provides an accurate and reliable indication of torque applied to a torque element.
Sensing of the magnetic field and specifically the axial components of the distortions in the magnetic field caused by torque is accomplished through the use of magnetic field sensors. A commonly used type of magnetic field sensors is a flux gate sensor, which is fabricated as a coil of fine wire surrounding a core of magnetically saturatable material, and is supplied with an alternating current. The alternating current provides for the periodic magnetic saturation of the magnetic elements. The magnetic field produced by the torque transducer shaft is superimposed on the periodic magnetic field generated by the coils. Superimposing the magnetic field produced by the torque transducer shaft creates an asymmetry in the magnetic saturation of the coils. Changes in the inductance of the coils due to magnetic saturation results in a voltage that is induced to the coils. It is this voltage that is measured to determine the amplitude and direction of torque applied to the torque transducer element.
A known prior art magnetic field sensor includes a bobbin having upper and lower axial sections separated by a central flange. The upper and lower coils are isolated from each other and are induced with an alternating current to produce a magnetic field. Magnetically saturatable strips are disposed between the coil and the torque transducer element. These magnetic strips are magnetically saturated by the alternating current that is produced within the coils. The magnetic strips are disposing parallel to the shaft and the axis of rotation. The magnetic strips are fabricated from a material that possesses a very abrupt magnetic saturation characteristic, meaning that the magnetic strips are saturatable through a small change in applied magnetic field and in the absence of the magnetic field quickly demagnetize.
Disadvantageously, manufacturing tolerances of each of the flux gates and magnetic asymmetries of the torque transducer and magnetoelastic materials can result in an incomplete cancellation of the external magnetic fields such as the earth's field effects.
Accordingly, it is desirable to design and develop a device and method for independently adjusting the sensitivities of each flux gate sections to permit precise cancellation of the effects of external magnetic fields.